1. Field of the Invention
The present invention relates to a modular boat for supporting semiconductor wafers of any diameter.
2. Discussion of the Related Art
The semiconductor component industry subjects silicon wafers to various treatments during their manufacturing. Such treatments require the use of boats, generally made of quartz or silicon, to vertically maintain a number of such silicon wafers. In order to optimize the cost of semiconductor components, it is necessary both to simultaneously treat as many wafers as possible and to reduce the time duration of some treatments, by increasing the temperature, for example.
There are several types of boats, commonly used for treatments of silicon wafers. Exemplary conventional boats are shown schematically in FIGS. 1a-1d.
The boat schematically represented in FIG. 1a is formed by a shell 1, in the shape of a hollow half cylinder, that includes grooves 2, along the concave portion of the shell. The boat securely maintains wafers having a determined diameter, as each wafer is held by engagement of its base in a circular groove. However, a drawback of such a boat is that it is difficult to form adequate and regular grooves on rounded surfaces. Additionally, this boat does not allow a satisfactory circulation of gas around the wafers.
The boat represented in FIG. 1b is formed by a grooved plane 3 having a central aperture. Each silicon wafer 4 is held vertically by its insertion into grooves 5 disposed on both sides of the central aperture. The grooves provide two small-surface supporting areas close to the base of the wafer.
FIG. 1c is a perspective view of a boat formed by two parallel rails 6, located along the same horizontal plane, that include notches 7, equally distributed along the two rails. A silicon wafer is vertically held by simultaneously engaging it into a pair of opposing notches. This gives the base of the wafer two supporting areas, the sizes of which corresponding to the depth of the notches.
A drawback of the boats represented in FIGS. 1b and 1c is illustrated by FIG. 1e. With such boats, the vertical positioning of the silicon wafers 4 is maintained by only two small supporting areas 18 disposed near the bottoms of the wafers, the sizes of such areas corresponding to the depths of the opposing grooves or notches. The silicon wafers may therefore become slightly slanted. If the distance between grooves is too small, this slanting may cause the upper parts of two adjacent silicon wafers to touch, producing poor quality wafers after treatment. Therefore, it is not possible using the boats of 1B and 1C to treat a large number of tightly spaced wafers at once, as there is a risk that the wafers will touch.
Last, the boat of FIG. 1d is made of four rails. The upper and lower planes, 8 and 9, respectively, of the boat are horizontal. Each of the planes includes a pair of parallel rails, 10 for the upper plane and 11 for the lower plane, the distance between the lower rails being smaller than the distance between the upper rails. The rails include notches that are regularly spaced along their lengths, so that a pair of upper notches 12 and a pair of lower notches 13 are positioned along a same vertical plane. The rail assembly is rigidly held together by horizontal rods 14 and oblique rods 15. A silicon wafer is vertically held by its engagement into two pair of notches, 12 and 13, positioned along a same vertical plane, thus providing four support areas, the sizes of which corresponding to the depths of the notches. The silicon wafers are thus better held in such a boat than in the two preceding boats.
A drawback of this last boat, and more generally of all of the prior art boats, is that all these boats are rigid. Any mechanical stress applied to them immediately transmits to the wafers, where such stress can deform the wafers and impair their manufacturing quality. Indeed, if a high-temperature process is required for specific treatment in order to shorten the duration of a treatment cycle, thus improving the production efficiency, all these boats become deformed due to thermal expansion. Because of their rigidity, these boats directly transmit their deformations to the wafers, which therefore become impaired. They can, for example, get warped.
To summarize, since the various known boats consist of a single piece and are rigid, when used to their maximum loading capacity or at high temperature, they impair the treatment of the silicon wafers, either by deforming them or by irregularly treating them. This impairment compromises manufacturing efficiency and incurs excessive cost for the semiconductor component industry.